Use of acyl thioureas in rubber



USE or ACYL THIOUREAS IN RUBBER Joseph C. Ambelang, Akron, Ohio, assignor to The Firestone Tire & Rubber Company, Akron, Ohio, a corporation of Ohio N Drawing. Application October 15, 1954 Serial No. 462,621

13 Claims. (Cl. 260-801) This invention relates to the use in vulcanized rubber compositions of a thiourea derivative of the formula in which R and R are the same or different alkyl groups containing up to nine carbon atoms or cyclopentyl, cyclohexyl, alkyl-substituted cyclopentyl and alkyl-substituted cyclohexyl groups, and in which R is an alkyl group of 1 to 5 carbon atoms.

The rubber compositions can be those used 'in tires, inner tubes, rubber thread, and other products produced ired States PatentO 2,822,412 Patented F eb- 4, -1 958 The following is illustrative of the preparation of the acyl thioureas useful in carrying out the invention:

ACETYL N,N'-DI-i-PROPYL THIOUREA One mole of N,N'-di-i-propyl thiourea was suspended in benzene and shaken while 1.1 mole of acetyl chloride was dropped in. Next was added 1.1 mole of pyridine. Water was then added to dissolve the pyridine hydrochloride. The benzene layer was washed with water and sodium bicarbonate solution. The acetyl thiourea crystallizedout on spontaneous evaporation of the benzene. After recrystallization from hexane, the product melted at 120l2l C. (corrected).

Analysis The melting points (corrected) and analyses for nitrogen (Kjeldahl) and sulfur (Carius) for a number of compounds prepared according to the forgoing general procedure are given in the following table:

from rubber latexes, and other rubber articles. The compositions consist essentially of natural rubber, or a sulfur-vulcanizable synthetic rubber, as for example,'a polymer of butadiene or an alkyl derivative thereof, or a copolymer of butadiene or an alkyl derivative thereof, or a copolymer of butadiene or an alkyl derivative thereof with a vinyl monomer, or a mixture of sulfur-vulcanizable rubbers.

The deterioration of rubber is due to different factors and is evidenced in various ways.

Although generally the acetyl disubstituted thioureas will be employed, because more readily obtainable commercially, the formyl, propionyl, butyryl, and vale'ryl derivatives can be employed. The disubstituted thioureas from which the antiozone agents of this invention are obtainable include, for example:

N,N-dimethyl thiourea N,N'-diethyl thiourea N,N'-di-n-propyl thiourea N,N'-di-i-propyl thiourea N,N-di-n-butyl thiourea N,N-di-t-butyl thiourea N,N-di-i-butyl thiourea N,N'-di-tt-octyl thiourea N-n-pentyl-N'-n-nonyl thiourea N-n-hexyl-N'-n-heptyl thiourea N,N-dicyclohexyl thiourea Oxygen and ozone both have a harmful effect on rub- 40 her, but the effect of each is different, and compounds which inhibit or prevent the harmful effect of one are not necessarily effective in stopping the harmful effect of the other.

Crabtree and Kemp in an article in Industrial. and Engineering Chemistry, vol. 38, starting at page 278 (1946) explain the difference in the action of oxygen and ozone. The light-catalyzed oxidation which occurs during outdoor exposure forms a skin and crazed appearance over the exposed surface of the rubber. Ozone, even in very low concentration, attacks stretched rubber only (C. H. Leigh-Dugmore, Rubber Age and Synthetics, November and December 1952) and forms cracks perpendicular to the direction of stretch, and such cracking can occur in the absence of light.

As a matter of fact, most commercial rubber antioxidants are without effect in inhibiting the deterioration caused by ozone.

Natural rubber is used in the manufacture of the sidewalls of tires. The cracking of such sidewalls has long been a problem. It is a primary object of this invention to provide antiozone agents which prevent such cracking. However, the antiozone agents of this invention are not limited to that use but may be employed in treads, thread and. other latex products, and other rubber products.

' The antiozone agents were tested in stock such as might be used in white sidewalls for tires. The control had the Natural rubber 100 White pigment and filler 90.2 Stearic acid 1.2 Sulfur 3.0 Accelerator 0.9

Total 195.3

known in the art.

sodium acetate and the like, as well as other activators Additionally, two or more accelerators or accelerator combinations are sometimes desirable in a single rubber compound. Many of the accelerators mentioned above are suitable in latex formulations, especially such common accelerators as piperidinium pentamethylene dithiocarbamate, zinc butylxanthate, zinc ethylxanthate, zinc salt of mercaptobenzothiazole, zinc dimethyldithiocarbarnate, and zinc dibutyldithiocarbamate, Although vulcanization is usually accomplished by heating a vucanizable rubber composition at a temperature in the range of 240 to 400 F. for a time ranging from several hours to a few seconds, vulcanization does take place at lower temperatures such as ordinary room tem- The results reported below include data on the tensile peraturea It is quite common to vulcanize a latex film properties of the cured rubber stocks before and after containing an ultra-accelerator by allowing the film to aging two days in an oven at 212 F. The modulus remain at room temperature for several hours or a few and tensile strength are given in pounds per square inch days, and the elongation is reported as percent of stretch at I l i the break. These data are included to show that the 1. A sulfur-vulcanized rubber composition which conantiozone agents have no substantial deleterious eifect rain a an inhibitor of ozone cracking an acyl thiourea on the cure or upon the aging of the cured stocks. having the formula After curing the stocks were exposed to natural R\TC S -NHR weathering for 62 days. The following table records 1 when the first cracks were observed in each and also the size and number of the cracks in each of the samples on in which R and R are from the class consisting of completion of the test: cyclopentyl, cyclohexyl, alkyl-substitutcd cyclopentyl,

Control 195. 3 195. 3 195. 3 195. 3 Aeetyl N,N'-di-i-propylthiourea 2 AcetylN,N-di-i-butylthiourea 2 Acetyl N ,N-dicyclohexylthiourea 2 Total 195. 3 197. 3 197. 3 197. 3

Normal Tensile Properties:

0 odulus 1,075 1,075 1,125 1,025

Tensile 3, 700 3, 300 3, 4.00 3, 525

Elongation 660 600 600 620 Aged 2 days at 212 F.:

400% Modulus.- 1,200 1,050 1, 150 1, 000

Tensile 2, 400 2, 000 2, 200 2, 375

Elongatlon 530 520 520 560 Natural Weathering Days to Initial Cracking 14 14 14 31 Crack rating at 62 days- Size Severe Moderate Moderate Slight Number Numerous Few Few o z Cracks Each of the antiozone agents reduced the size of the alkyl-substituted cyclohexyl and alkyl groups containing cracks very materially. Also the number was decreased. l to 9 carbon atoms, and R is an alkyl group containing In the test with acetyl N,N'-dicyclohexylthiourea, signs of l to 5 carbon atoms. incipient cracking appeared on the edges, where cracking 2. The composition of claim 1 in which R in the has been found to be an inverse function of the sharpinhibitor is methyl. ness of the die with which the dumbbell strips are cut. 3. The composition of claim 1 in which R in the By sulfur-vulcanization is meant the curing of rubber inhibitor is ethyl. by reaction with either free sulfur or a vulcanizing agent 4. The composition of claim 1 in which R in the of the sulfur-donor type. Known agents of the latter inhibitor is propyl. type include the various phenol polysulfides including the 5. The composition of claim 1 in which R in the alkyl derivatives thereof, the xanthogen polysulfides, the inhibito is butyl, thiuram disulfides and polysulfides, various amine sul- 6. The composition of claim 1 in which the inhibitor fides including the dialkylamine polysulfides and reaction 0 is acetyl N,N-di-i-propylthiourea. products of primary amines with excess sulfur. Known 7. The composition f laim 1 in hi h the i hibit vulcanization accelerators are useful in speeding up the is acetyl N,N'-di-n-b tylthi ea vulcanization process and operative herein, especially 8. The composition of claim 1 in which the inhibitor the relatively active accelerators including the thiazole i aoetyl N,N'-dicyclohexylthjourea sulfenamides, e. g., Ncyclohexyl-2-benzothiazolesulfen- 9, Th meth d f vulcanizing bbe which comprises amide, thiazoline sulfenamides, thiocarbamyl sulfenamsulfur-vulcan-izing the same in the presence of an acyl ides, mercaptothiazoles, mercaptothiazolines, thiazolyl thiourea having the formula monoand di-sulfides, the N,N-disubstituted dithrocar: bamates, the thiuram sulfides, and xanthogen sulfides, and RNC(S)-NHRi metallic salts of mercaptothiazoles or mercaptothiazolmes or dithiocarbamic acids.

One or more accelerator activators is often used with Whldl R and 1 from i class consisting of any of the accelerators mentioned, and such activators Y P WL Y YL alkyl-substltllted Y PF YL alkyl include the various derivatives of guanidine known in the su fl cyclohexyl d alkyl groups contalmna 1 t rubber art, amine salts of inorganic and organic acids, 9 on atoms, and s 18 an alkyl group Contammg 1 various amines themselves, and alkaline salts such as to 5 carbon atoms.

V is an acetyl thiourea.

11. The method of claim 9 is which the acyl thiourea is acetyl N,N-di-i-propylthiourea.

12. The method of claim 9 in which the acyl thiourea 5 is acetyl N,N'-di-nbutylthiourea.

13. The method of claim 9 in which the acyl thiourea is acetyl N,N-dicyclohexylthiourea.

UNITED STATES PATENTS Vegvar Oct. 4, 1949 '6 Beaver Aug. 4, 1953 Brown Dec. 15, 1953 Butler Jan. 5, 1954 Kendall Nov. 23, 1954 Hotten Dec. 28, 1954 Huebner Mar. 8, 1955 Huebner Aug. 2, 1955 OTHER REFERENCES Moore: Journal Am. Chem. Soc., 62, 3273-4 (1940).

U. S. DEPARTMENT OF COMMERCE PATENT OFFICE CERTIFICATE OF CQRREC TIQN Patent No, 2,822,412 February 1958 Joseph C, Ambelang It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, lines 73 and '74, for eyeloperryl, alkyl substituted" read --cyclopentyl, alkyl-substituted==.,

Signed and sealed this 1st day of April 1958,

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Officer Conmissioner of Patents 

1. A SULFUR-VULCANIZED RUBBER COMPOSITION WHICH CONTAINS AS AN INHIBITOR OF OZONE CRACKING AN ACYL THIOUREA HAVING THE FORMULA 